An internal combustion engine may be equipped with a variable valve actuation (VVA) system that provides for selective adjustment of the timing of engine valves to improve engine efficiency. The VVA system may be controlled to selectively override valve timing provided by a conventional cam-driven actuation system. For example, the timing of the intake and/or exhaust valves may be modified by the VVA system to implement a variation on the typical diesel or Otto cycle known as the Miller cycle. In a “late intake” type Miller cycle, the intake valves of the engine are held open during a portion of the compression stroke of the piston. This may result in an improvement in the overall efficiency of the engine.
Conventional VVA systems are hydraulically powered. Accordingly, changes in properties of the fluid used to operate the VVA systems may change the operation of the systems. For example, an increase or decrease in a rate at which fluid flows the system may result in an increase or decrease in the time required for the system to operate. In particular, when the engine is starting, the operating fluid may have a cold temperature and, thus, a high viscosity. The high viscosity of the fluid increases the amount of time required for the system to operate. This increased operation time may unexpectedly change valve timing and reduce or eliminate any performance gains that may have been achieved by implementing a variation on conventional valve timing. For example, the highly viscous fluid within the valve actuator may hold the intake valves open longer than a desired time, making the engine difficult to start.
One method of solving problems associated with a high viscosity fluid in a hydraulic valve actuation system is described in U.S. Pat. No. 6,976,459 (the '459 patent) issued to Kagy et al. on Dec. 20, 2005. The '459 patent describes a method of controlling an engine having a valve actuation system. The method includes sensing first and second temperatures upon cold start-up of the engine. When the first and second temperatures are a below predetermined value, the valve actuation system is disabled to prevent a variation on conventional engine valve actuation timing.
Although the system and method of the '459 patent may solve some problems associated with cold start-up of an internal combustion engine, they may still be less than optimal. For example, disabling of the valve actuation system in the '459 patent may lead to undesirable emissions and reduce the overall efficiency of the engine until the first and second temperatures rise above the predetermined value.
The VVA system of the present disclosure is directed towards overcoming one or more of the problems set forth above and/or other problems of the prior art.